Atomic-Scale Phase Transformation Mechanisms and Microstructural Evolution of BCC-Fe and Fe–C Alloys under Transient Excitation

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Abstract
Martensitic transformation in rail steel is closely associated with surface damage and microstructural degradation under wheel–rail interaction. To clarify the underlying mechanisms, phase transformation in Fe and Fe–C alloys under coupled mechanical and thermal loading was investigated using LAMMPS-based molecular dynamics simulations and a self-developed fatigue impact test system. The results show that polycrystalline structures exhibit lower transformation stresses than single crystals and are more susceptible to martensitic transformation. Stress concentration and carbon segregation at grain boundaries promote the nucleation of new phases. Under thermomechanical coupling, increasing carbon content decreases the austenitic transformation temperature and reduces phase stability. High excitation energy induces lattice distortion and generates vacancies and dislocations, which serve as rapid diffusion pathways for interstitial atoms. Repeated excitation improves surface morphology, refines grain structure, and enhances phase homogeneity. The microstructure is dominated by low-angle grain boundaries, with a high-angle to low-angle grain-boundary ratio of 0.679. By integrating atomic-scale simulations with excitation experiments, this study provides new insights into rail surface damage and phase-transformation evolution, offering theoretical support for mitigating transformation-related damage in railway applications.

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